In conventional tube and fin heat exchangers such as motor vehicle engine radiators where the tubes have a flat or oval shape cross section for minimum resistance to air flow through the core, there is a tendency for the tubes to balloon with pressure. For example, such flat tube heat exchangers must be structurally designed to withstand internal pressures during the coolant system filling operation at vehicle assembly as well as system pressures intentionally developed to increase coolant boiling point during vehicle operation. Under such pressurization, the unsupported flat or oval tube walls are subjected to severe bending movements developed from the internal pressure acting on the flattened portion of the tube. These bending moments are resisted until tensile stresses in the tube wall exceed the elastic material limit after which the tube plastically deforms and seeks a circular cross section. If a circular cross section is achieved before the tube bursts, the bending moment disappears stabilizing the cross sectional contour and leaving only circumferential tension and radial shear stresses. However, such ballooning then adds to the air flow resistance.
Moreover, wide flat tubes are known to improve the economics of tube and fin radiator designs by delaying the need for multiple tube rows in deeper cores and by increasing the overall thermal core efficiency by maximizing direct tube fin contact area. However, the bending moment mentioned above becomes more severe as tube width increases. Furthermore, many mass produced tube and fin heat exchangers maintain a gap between the header and the first fin convolution so as to provide a clearance to repair tube and/or header leaks in an assembled heat exchanger. However, as the length of the unsupported tube portion increases, the gap further amplifies the tube support problem by exposing unsupported tube length between the header and first fin convolution. And to maintain the flat or oval configuration under internal pressure, the tube depends strongly on the column support provided by the traversing fins. For example, an internally pressurized flat tube and fin heat exchanger with a gap between the header and first fin convolution typically experiences a tube ballooning problem that begins with a portion of unsupported tube between the header and first supporting fin convolution and propagates axially along the tube length. As the length of unsupported tube decreases, the threshold ballooning pressure increases to a value reflecting the maximum available support provided by a series of continuous columns, i.e. the fin convolutions. And thereafter, the tube ballooning problem will then begin in a random location within the core. Relating this to mass production, radiator families for example using similar tube designs are typically produced in several fin densities to offer a range or performance tailored to economically satisfy specific applications. But as a result due to the increased tube support per unit length, otherwise similar cores with increased fin density typically suffer tube ballooning problems as described above but at a higher threshold pressure level. And thus a minimum internal pressurization design specification must be withstood by the lowest fin density core within a family of heat exchangers using the same tube design. Due to section modulous considerations, a fin column is strongly dependent upon fin gage or thickness. And in high volume manufacturing, material gage consistency simplifies manufacturing tracking efforts and minimizes the potential for mixed parts. Fin gages are then constrained to assure that the minimum internal pressurization specification is met by the lowest fin density heat exchanger within a family utilizing the same tube design. However, the fin gage consistency constraint imposes a substantial fin material penalty on higher fin density cores which offer more tube support per unit length than their low fin density counterparts.